Blind rivets are widely used in closed aeronautical structures, where attachments have very restricted access on one of the sides, with no possibility of accessing or even viewing the rear part of the attachment. Consequently, the rivet quality evaluation process is carried out exclusively from the access face (‘exposed face’).
Blind rivets are inserted in the corresponding holes, and the riveting process is carried out until the stem of the rivet breaks, the rivet being installed. However, since accessing the blind face (or unexposed face) is not possible, it is not possible to carry out a complete quality control of the attachment.
There is no absolute assurance that the inspection method supplied by rivet manufacturers detects faults inside the rivet or in its closing head (unexposed face) when the choice of rivet is not suitable according to the thickness of the attachment, reaching a critical situation when the thickness is close to the limit between two lengths of the rivet.
Although the process parameters leading to the correct installation of rivets are known in the vast majority of installations, and despite the current control methods, a significant number of attachments are still produced in which the rivets are incorrectly installed. Consequently, the attachments designed by means of blind rivets must be oversized, increasing the number of necessary rivets, manufacturing times, weight and cost of the resulting attachments. Furthermore, the existing methods based exclusively on monitoring the process do not contemplate errors due to machining steps after the riveting itself, such as machining the mandrel, a necessary operation in aeronautical applications for meeting the aerodynamics requirements.
A system which, in a non-destructive way, informs of the correct installation of rivets, it is necessary to assure the correct installation of the rivets with maximum assurance.
Until now methods for inspecting the state of the rivet have been proposed. Most of the proposed methods are based on monitoring the riveting process. These methods are based on measuring one or several signals during the riveting process and subsequent analysis thereof, either for comparing with standard curves or for extracting evaluation values which are subsequently compared with reference values. Some of the existing methods based on monitoring the process are listed below. First, document U.S. Pat. No. 5,666,710A describes monitoring the “force” and “displacement” signals and the evaluation value is the energy invested in the riveting process. In patent U.S. Pat. No. 7,313,851B2 the same signals are monitored but the evaluation value is an instant comparison with the standard curve characterising installation processes. Document WO2007028218A1 describes monitoring the torsion angle in the rivet.
These methods based exclusively on monitoring the process do not contemplate errors due to machining steps after the riveting itself, which steps are necessary in constructing riveted structures and can induce defects in the rivets.
Unlike methods based on monitoring the riveting process, the method proposed in GB2425179 is to be applied once the rivet is completely installed, and is intended for analysing riveted attachments of the same material or different materials. Said method is based on analyzing the electrical impedance of the rivet. It consists of measuring the electrical impedance of the rivet to be evaluated by means of using an ultrasonic generator in a narrow bandwidth. Both the real and complex electrical impedance measured are compared with previously established reference thresholds.
The main technical problem which the invention solves is that of finding a technical solution to the contactless evaluation of blind rivets installed in closed structures or structures for which it is complicated or costly to access the “unexposed face”, based on the analysis of the riveting step and the subsequent evaluation for immediately classifying the rivet as suitable or unsuitable.